Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. One or more streamer cables containing acoustic and/or electromagnetic (EM) seismic receivers are deployed into the water behind a vessel, and one or more sources may be towed by the same or different vessel. Less than perfect knowledge of the actual positions of the source at the time of firing and receivers at the time of arrival of reflected seismic waves may result in less than acceptable seismic data.
When performing marine seismic imaging of the subsurface strata one needs to establish the position and depth of the seismic source and the receivers (either acoustic or EM). Conventionally they have been referenced to the sea surface, but this has the disadvantage that the actual surface varies up and down with time and it is thus at a different distance from the seabed at the different times of the seismic experiments.
The Global Positioning System (GPS), administered by the United States, is a satellite-based positioning system useful in marine seismic exploration, and seismic surveys may employ multiple GPS receivers at strategic points in a spread to determine the surface position of a vessel, or buoys tethered to streamers and sources. However, this still does not provide knowledge of the actual position of the receivers on the streamers and the sources, as they are underwater and not at the surface. Thus, GPS has been used for surface positioning in marine seismic data acquisition, but one must still accurately relate the spatial position of GPS receiving antenna to underwater acoustic or EM equipment. Patent Cooperation Treaty publication no. PCT/WO/03/100451 A2 discloses a GPS-based underwater cable positioning system including a plurality of towed surface units and streamers. Each surface unit has a GPS receiver for determining its position, and an acoustic transmitter to transmit a signal representative of its position into the water. As described, acoustic receivers on the streamer cables receive the signal from the near surface transmitters and determine their position from the signals. These methods and systems can be expensive as each receiver must include the function of calculating its position from the signals it receives from the acoustic transmitters on the towed surface units; also, if one fails, the position data for that receiver is lost and that receiver (or the entire streamer cable) must be replaced, contributing to downtime and/or the need for further surveys.
Currently there are several ways used to relate an acoustic or EM measuring device to a satellite or radio antenna in seismic positioning networks. These include compass directions that represent the direction between the acoustic device and the antenna and an assumed (known) distance. In other previously known methods, an acoustic or EM transponder is affixed directly on the GPS buoy, or on a vertical pole extending into the water from the buoy having the GPS antenna, or towed by the buoy in a “fly behind” arrangement. In the aerospace and maritime industries it is common to combine high precision relative GPS signals between three antennae with a fixed baseline between them with an inexpensive solid-state component inertial system to sense 3D motion. GPS/inertial is an inexpensive and smaller equipment set than typical vessel gyros. The inexpensive inertial units have much higher drift rates than mechanical gyros but the drift is bounded by the high recalibration rate available from GPS. These methods have until now been adequate since the precision of satellite systems such as the GPS have been in the same neighborhood as the precision of the connections mentioned above, e.g., a few meters. Previous to the present disclosure, the initial position to within few meters of accuracy of an underwater component could be determined for instance by using GPS combined with an acoustic positioning system, such as a short-baseline (SBL) or ultra-short baseline (USBL) acoustic system. However, satellite-based systems have recently been largely improved in terms of precision. Methods of the present invention seek to take advantage of this improvement to relate spatial position of the satellite antenna to the underwater acoustic or electromagnetic device on the satellite level of precision, e.g., sub-meter, during one or more seismic and/or EM surveys.